Shortly before one o'clock on Tuesday I was ­awaiting a friend for lunch in the Atrium, a cavernous Westminster eatery frequented by politicians, civil ­servants and journalists. Instead of my guest, an MP well-known for his forceful criticism of Islamism, there appeared a gaggle of ­Islamists, dressed in salwar kameez-style overshirts and trousers, with designer skullcaps and some bushy black beards to match. Talking excitedly on their mobile phones, they hung up a black banner proclaiming "Sharia for the UK" in a room at the back of the restaurant – for the benefit of the television cameras gathering for a press conference. Beside it they pasted neatly lettered posters saying "Islam will Dominate the World", "Democracy is Hypocrisy" and, as if to trump the slogans in Orwell's 1984, "Freedom = Dictatorship".

At one o'clock, their leader, Anjem Choudary, sat down in front of the microphones to explain why the ban on Islam4UK announced that morning by the home secretary was "a victory for Islam and Muslims". With the fluency of your media-savvy professional extremist, he steered just on the safe side of the law while producing telly-friendly outrageous soundbites such as accusing the British government of "state-sponsored terrorism". His remarks were interrupted by occasional cries of "Allahu akhbar" from a small group – perhaps 10 to 15 at most – of his designer-Islamist followers. Their neatly rehearsed chants were not loud enough to disturb anyone's lunch in the restaurant outside, but doubtless sounded suitably Talibanesque on television. As I stood at the door to the press conference, watching these young men excitedly whispering to each other while texting on their mobiles, they seemed to me closer to the celebrity-seeking culture of British TV's Big Brother and The X Factor than to the world of the Islamic caliphate, whose restoration they demand – or at least, they weirdly combined the two.

Choudary, this would-be Simon Cowell of Islamist impresarios, defended Islam4UK's suggestion that it might hold a protest in Wootton Bassett. (Probably never a serious plan, this was a hugely successful publicity stunt, with the prime minister, no less, rising to the bait). He denounced the "hypocrisy of man-made law". And he said the ban – which is supposed to come into force today – is "the biggest favour the British government could do us".

I fear he may be right. An ineffective ban will be the worst of both worlds, and we know that the ban is likely to be ineffective, because Islam4UK itself emerged when two other offshoots of the original al-Muhajiroun organisation were banned – and Choudary has more or less said this is what he'll do again. The fungus will re-emerge under a different name, or none. If Choudary carries on as he is now, reportedly living on state benefits provided by the hard-pressed British taxpayer, enjoying police protection against possible right-wing attacks, yet spitting his practised venom to the media – now with the added celebrity that the prime minister and the home secretary have given him – then it will be worse than if he had all along been ignored. How much better to dismiss this man, in traditional British fashion, as a joke and a jerk.

Moreover, the timing of the ban makes it look like party political opportunism. Yes, the Home Office claims that it results from a thorough analysis by the Joint Terrorism Analysis Centre, only recently submitted to the home secretary. But it's not as if these people appeared yesterday. If any of them are terrorists, they should have been arrested long ago. If others are recruiters or cheerleaders for terrorism, they have been that for some time. So why now? Any politically literate citizen will say: well, after the row over the proposed Wootton Bassett demo and in the run-up to an election, Labour was desperate not to be wrongfooted by the Conservatives, who have already said they will ban the much more influential Islamist organisation Hizb ut-Tahrir. Don't let the Daily Mail nail you on this one, goes the Alastair Campbell-style calculation. You don't need to worry about losing many Muslim votes over this guy, so cover your backside on the right.

Talking of Hizb ut-Tahrir, I have just read an interesting report from the Centre for Social Cohesion, a thinktank no one could accuse of being soft on Islamism, which argues that the best response to Hizb ut-Tahrir is not the Tories' proposed ban but rather a strategy of "civic and civil intolerance". In other words, its representatives should be treated and combated as pariahs, like leaders of the BNP. I don't agree with all the authors' individual recommendations, but the thrust of their analysis is compelling.

So this argument has nothing to do with trivialising the threat. It has ­everything to do with identifying it clearly and fighting it effectively. In the last few weeks, we have twice been reminded exactly what the threat is: Islamist violence and the broader intimidation that results from the credible threat of violence. A young Islamist tries to bring down an airplane over Detroit. Another tries to murder the Danish cartoonist Kurt Westergaard in his own home with a knife and an axe. It is shocking to realise just how many people in Europe now live in fear for their lives.

The trouble is that nobody knows which of the many other alienated young Muslim men out there will become so radicalised that he will turn into a bomber or axeman, and which of them, given the right conditions, will gravitate into the mainstream of ­society. Perhaps in 20 years' time, one of the young Islam4UK activists I rubbed shoulders with in that back room at the Atrium will be dining in the main ­restaurant, as a spin doctor for the Tories. Or perhaps in 20 months' time he'll be trying to detonate a bomb; and, unlike the Detroit bomber, he may not be stopped in time.

But if you ­discriminate against a whole social group, you not only violate the basic principles of a free society, you may also achieve the opposite of the desired effect. A recent report by the ­Quilliam Foundation produced ­alarming ­evidence of how some people are ­actually being turned into ­violent ­Muslim extremists inside British prisons.

So: we must keep our sights sharply focused on the target, which is violence. The world is full of ­people believing and saying ­idiotic, outrageous, hateful and ­offensive things. If we locked them all up, half of humankind would be guarding the other half. What matters is what leads to ­violence. To stop the descent to violence, we need many different sorts of action, some ­apparently soft, some obviously hard.

When such actions do not erode liberty, and have some prospect of success, there is no harm in trying them. When they do erode liberty, however, like a ban on a political organisation or a restriction on free speech, then you need to have compelling evidence that they will bring a real gain in security – and I don't mean security for one political party against electoral attack from another. What we cannot afford, and what we have seen too much of over the last decade, is steps that reduce liberty without increasing security.